Injection molding enables production of a molded article having a complicated shape with high productivity, and thus is widely spread for machine parts, automobile parts, electric and electronic parts, food and drug containers and the like. In recent years, various plastic containers have been used as packaging containers owing to the advantages thereof, such as light weight, transparency and good moldability. As a representative plastic container, for example, a container for beverages and the like, an injection molded article having a mouth with a screw shape enabling sufficient plugging with a cap has been frequently used.
Examples of the material used in the injection molded article include a polyolefin, such as polyethylene and polypropylene, and a versatile resin, such as polyester and polystyrene. In particular, an injection molded article using a plastic container (such as a bottle) mainly containing a polyester, such as polyethylene terephthalate (PET), has been widely used for beverages, such as tea, juice beverages and carbonated beverages. The injection molded article mainly containing a thermoplastic resin is excellent as a packaging material, but as being different from a glass bottle and a metal container, it has such a nature that oxygen penetrates from the outside thereof, and thus has a problem remaining in storage stability of the content charged and sealed therein. For imparting the gas barrier property to the injection molded article containing a versatile resin, a multilayer injection molded article having a gas barrier layer as an intermediate layer has been subjected to practical use.
Now a polyamide obtained by polycondensation reaction of xylylenediamine and an aliphatic dicarboxylic acid, for example, a polyamide obtained with m-xylylenediamine and adipic acid (which is hereinafter referred to as nylon MXD6), exhibits a high strength, a high elastic modulus, and a low permeability to a gaseous substance, such as oxygen, carbon dioxide, odors and flavors, and thus is widely used as a gas barrier material in the field of packaging materials. Nylon MXD6 has good heat stability on melting, as compared to other gas barrier resins, and thus can be co-extruded or co-injection molded with a thermoplastic resin, such as polyethylene terephthalate (which is hereinafter abbreviated as PET), nylon 6 and polypropylene. Accordingly, nylon MXD6 is utilized as a gas barrier layer constituting a multilayer structure.
However, even in the case where nylon MXD6 is used as the gas barrier layer, not only oxygen cannot be blocked completely due to the slight oxygen permeability thereof, but also oxygen remaining in the gas in the head space present above the content after charging in the molded article cannot be removed. Accordingly, deterioration of contents that are sensitive to oxygen, such as beer, cannot be avoided. Furthermore, a multilayer molded article obtained by biaxially stretching blow molding of a parison having a three-layer structure of PET resin layer/nylon MXD6 resin layer/PET resin layer or a five-layer structure of PET resin layer/nylon MXD6 resin layer/PET resin layer/nylon MXD6 resin layer/PET resin layer has a problem that it may be more likely to suffer interlayer delamination upon application of impact or upon rapid pressure change after charging a carbonated beverage.
As another measure for imparting the gas barrier property to the injection molded article containing a thermoplastic resin, such a method has been practiced that a gas barrier layer containing an ethylene-vinyl alcohol copolymer, polyacrylonitrile, polyvinylidene chloride, an aluminum foil, a carbon coating, an inorganic oxide vapor deposit, or the like is laminated as a constitutional material, but as similar to the case using nylon MXD6, oxygen remaining in the gas in the head space present above the content after charging in the molded article cannot be removed.
In recent years, a small amount of a transition metal compound is added and mixed with nylon MXD6 to impart an oxygen absorbing capability to nylon MXD6, which is utilized as an oxygen barrier material constituting a container or a packaging material, whereby oxygen penetrating from the outside of the container is absorbed by nylon MXD6, and simultaneously oxygen remaining inside the container is also absorbed by nylon MXD6, thereby further enhancing the storage stability of the content of the container as compared to a container utilizing an ordinary oxygen barrier thermoplastic resin (see, for example, Patent Documents 1 and 2).
An oxygen absorbent has been steadily used for removing oxygen in a container. For example, Patent Documents 3 and 4 disclose an oxygen absorbing multilayer material containing an oxygen absorbent, such as iron powder, dispersed in a resin. Patent Document 5 discloses a product containing an oxygen removing layer containing an ethylenic unsaturated compound, such as polybutadiene, and a transition metal catalyst, such as cobalt, and an oxygen barrier layer, such as polyamide.
In the method of laminating the gas barrier layer as a constitutional material, interlayer delamination and cracks may occur upon application of impact. The formation of interlayer delamination or cracks may cause deterioration of the gas barrier capability, which may impair the commercial value. For addressing the issue, Patent Document 6 discloses improvement of the interlayer delamination resistance in such a manner that upon injecting the resins constituting the innermost layer and the outermost layer into the cavity of the mold finally, a coarsely mixed resin is made to intervene between the layers to form a preform by using a back-flow controlling device capable of making back-flow in a certain amount to the side of the gas barrier layer.